1. Field of Invention
The present invention relates to an optical electronic technology. More particularly, the present invention relates to a diffractive stack pickup head for an optical disk drive and a method to fabricate the diffractive stack pickup head.
2. Description of Related Art
Pickup heads are an opto-electronic means by which a signal is written to or read from an optical disk. FIG. 1 is a system drawing, schematically illustrating a conventional optical disk pickup system. In FIG. 1, the generic optical disk pickup system includes a laser diode light source 20, a beamsplitter 22, and at least one lens 24 for precisely focusing the light from the laser diode 20 onto an information bearing surface of the optical disk 26. The system further includes a multi-element photodetector 28 for receiving light that is reflected from the disk 26, and a servo-generating element 30, such as the cylindrical lens, which is used to alter the reflected beam from the disk 26 into some form whereby the state of focus and tracking can be easily extracted from the signals generated by the light incident on the multi-element photodetector 28. This conventional system can be referred to A. B. Marchant, Optical recording, A Technical Overview, Addison-Wesley Publishing, Menlo Park Calif., 1990, p.197. The general mounting arrangement for these optical elements into a pickup head unit is shown in FIG. 2. In FIG. 2, each component is mounted separately, one at a time, into a pickup head housing 32. The size of the housing 32 is directly related to the image distance of the focusing lens(es) 24, i.e., the distance from the laser diode 20 to the focused image of the laser diode 20 on the disk 26 of FIG. 1. With current technology of refractive molded aspheric lenses, this distance is typically in the range of 25-40 mm, and the size of the housing is roughly of the same order. In some designs, the servo-generating element can be an integral part of the beamsplitter 22 and sometimes is implemented using a diffractive or holographic optical element (DOE). FIG. 3 is a pickup system drawing, schematically illustrating an alternate generic pickup design where the beamsplitter and servo-generating elements are combined into a single DOE and the complete optical system excluding the objective lens is packaged in a small module. In FIG. 3, the design of the pickup system 38 has the advantage that most of the opto-electronic components 40, except the focusing lens/actuator 42, are assembled in a miniature module. The components 40 are also assembled one at a time to form the miniature module. To complete the pickup system, the components 40 are mounted in a pickup housing similar to the one shown in FIG. 2. However, the general size of the pickup system 38 is still determined by the imaging distance of the focusing lens. In other words, the holographic module approach has certain advantages but it does not significantly change the size or weight of the complete pickup system.
The invention provides an optical disk pickup head with the current state-of-the-art technology having an objective that the invention allows many pickups to be made simultaneously in parallel, thereby significantly reducing manufacturing cost.
The invention also includes another objective that the entire pickup, including the focusing lens, can be made in a very small dimension, such as a few millimeters on a side. This allows multiple pickups to be utilized in a single drive.
As embodied and broadly described herein, the invention provides a pickup head used in an optical data storage system. The pickup head includes a diffractive optical element (DOE) stack on a semiconductor substrate. The DOE stack includes multiple diffractive lenses for providing several diffractive surfaces and a middle layer serving as a beamsplitter and servo-generating element for a light reflected from an optical storage medium. The middle layer is sandwiched by the diffractive lenses that are located on both outer parts of the DOE stack. The semiconductor substrate includes at least a laser source and several photodetectors.
The middle layer""s function preferably includes a polarization-selective DOE and a quarter-wave retarder oriented to rotate the polarization state of the laser source, so that only the light reflected from the optical storage medium is diffracted by the polarization-selective DOE.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.